Many liquid hydrocarbons are comprised of various fractions which- vaporize under atmospheric or subatmospheric pressure, at different temperatures. In typical practice, such hydrocarbons are fractionated by heating and vaporizing one or more of such fractions to separate the lighter, lower boiling range fractions from the heavier, higher boiling range material. Since the process of fractionation separates the lighter, lower boiling range fractions as vapors, certain constituents of the hydrocarbons which do not vaporize remain in the heavier, higher boiling range portion of the hydrocarbon. Examples of constituents with this characteristic include metals, asphaltenes (pentane-insolubles), and coke pre-cursors, such as those measured by ASTM test procedures D-189 and designated as Conradson Carbon Residue (CCR).
These constituents are a problem for a variety of potential users of the heavier, higher boiling range portion of the hydrocarbon. Examples of users for whom such constituents present a problem include power generation devices, such as combustion turbines and internal combustion engines, and refinery process units such as catalyst-based cracking and hydrotreating units and thermal cracking units.
An example of a user that is effected by the constituents present in the heavier, higher boiling range portion of oil is the combustion turbine, which is one of the lowest cost, highest efficiency power generation systems available today. Combustion turbines can also be configured in a combined cycle configuration to further increase the efficiency of a power generation cycle. Combustion turbines can be damaged when using liquid fuels that contain significant amounts of metals. To avoid such damage, users of combustion turbines can: (1) use fuel with low levels of metals, (2) use fuel pre-processing systems to reduce the level of metals in the fuel burned, (3) add chemicals to the fuel to reduce the negative impacts of the metals in the fuel, or (4) operate the combustion turbine at a lower, and less efficient firing temperature to reduce the impact of the metals. Each of these options results in an increased cost of power generation, whether from additional capital, additional operating costs, or lower power generation efficiency.
One of the lowest cost liquid fuels available for use in combustion turbines is heavy fuel oil. Such oil is produced by mixing the heavier, higher boiling range portion of the hydrocarbon with sufficient light petroleum diluent, e.g., diesel fuel, to achieve the desired product properties. While the resultant heavy fuel oil usually has a lower cost than other liquid fuel products, the level of metals in the oil is usually higher, causing higher operating and maintenance costs and lower power generation efficiencies in combustion turbines. Moreover, such heavy fuel oil contains some amount of light petroleum product as diluent and the diluent alone has a higher value than that of the heavy fuel oil.
Conventionally, the low quality of the heavy fuel oil can be improved prior to use by fuel treatment systems such as centrifuging or settling to remove sediment, water washing to remove water soluble corrosive salts, and the addition of inhibitors to control the effect of non-removable corrosive elements.
The cost of heavy fuel oil can be reduced by purchasing a lower quality product, which then requires the use of a greater amount of fuel treatment, which results in lower combustion turbine efficiency and increases downtime.
In U.S. Pat. No. 4,191,636, heavy oil is continuously converted into asphaltenes and metal-free oil by hydrotreating the heavy oil to crack asphaltenes selectively and remove heavy metals such as nickel and vanadium simultaneously. The liquid products are separated into a light fraction of an asphaltene-free and metal-free oil and a heavy fraction of an asphaltene and heavy metal-containing oil. The light fraction is recovered as a product and the heavy fraction is recycled to the hydrotreating step.
In U.S. Pat. No. 4,528,100, a process for the treatment of residual oil is disclosed, the process comprising the steps of treating the residual oil so as to produce a first extract and a first raffinate using supercritical solvent extraction, and then treating the first raffinate so as to produce a second extract and a second raffinate again by supercritical solvent extraction using a second supercritical solvent and then combining the first extract and the raffinate to a product fuel. In accordance with a particular embodiment of the invention disclosed in the U.S. Pat. No. 4,528,100, the supercritical solvents are particularly selected to concentrate vanadium in the second extract. Thus, even though the amount of vanadium present in the product fuel is low and consequently beneficial for reducing gas turbine maintenance problems as stated in this U.S. Pat. No. 4,528,100, some amount of vanadium does still remain therein.
Another example of a user of the heavier, higher boiling range portion of a hydrocarbon is a refinery with a fluid catalytic cracking unit (an FCC unit). FCC units typically are operated with a feedstock quality constraint of very low metals asphaltenes, and CCR (i.e., less than 10 wppm metals, less than 0.2 wt % asphaltenes, and less than 2 wt % CCR). Utilization of feedstocks with greater levels of asphaltenes or CCR results in increased coke production and a corresponding reduction in unit capacity. In addition, use of feedstocks with high levels of metals and asphaltenes results in more rapid deactivation of the catalyst, and thus increased catalyst consumption rates and increased catalyst replacement costs.
In U.S. Pat. No. 5,192,421, a process for the treatment of whole crude oil is disclosed, the process comprising the steps of deasphalting the crude by first mixing the crude with an aromatic solvent, and then mixing the crude-aromatic solvent mixture with an aliphatic solvent. The U.S. Pat. No. 5,192,421, (at page 9, lines 43-45) identifies that certain modifications must be made to prior art solvent deasphalting technologies, such as that described in U.S. Pat. Nos. 2,940,920, 3,005,769, and 3,053,751 in order to accommodate the process described in the U.S. Pat. No. 5,192,421, in particular since the prior art solvent deasphalting technologies have no means to remove that portion of the charge oil that will vaporize concurrently with the solvent and thus contaminate the solvent used in the process. In addition to being burdened by the complexity and cost resulting from the use of two solvents, the U.S. Pat. No. 5,192,421 process results in a deasphalted product that still contains a non-distilled portion with levels of CCR and metals that exceed the desired levels of such contaminants.
In U.S. Pat. No. 4,686,028 a process for the treatment of whole crude oil is disclosed, the process comprising the steps of deasphalting a high boiling range hydrocarbon in a two-stage deasphalting process to produce separate asphaltene, resin, and deasphalted oil fractions, followed by upgrading only the resin fraction by hydrogenation or visbreaking. The U.S. Pat. No. 4,686,028 is burdened by the complexity and cost of a two-stage solvent deasphalting system to separate the resin fraction from the deasphalted oil, in addition, like the U.S. Pat. No. 5,192,421, the U.S. Pat. No. 4,686,028 process results in an upgraded product that still contains a non-distilled fraction--the DAO--that is contaminated with CCR and metals.
In U.S. Pat. No. 4,454,023 a process for the treatment of heavy viscous hydrocarbon oil is disclosed, the process comprising the steps of visbreaking the oil; fractionating the visbroken oil; solvent deasphalting the non-distilled portion of the visbroken oil in a two-stage deasphalting process to produce separate asphaltene, resin, and deasphalted oil fractions; mixing the deasphalted oil with the visbroken distillates; and recycling and combining resins from the deasphalting step with the feed initially delivered to the visbreaker. The U.S. '023 patent is burdened by the complexity and cost of a two-stage solvent deasphalting system to separate the resin fraction from the deasphalted oil. In addition, the '023 process results in an upgraded product that still contains a non-distilled fraction--the DAO--that is contaminated with CCR and metals.
In U.S. Pat. No. 5,601,697 a process is disclosed for the treatment of topped crude oil, the process comprising the steps of vacuum distilling the topped crude oil, deasphalting the bottoms product from the distillation, catalytic cracking of the deasphalting oil, mixing the distillable catalytic cracking fractions (atmospheric equivalent boiling temperature of less than about 1100.degree. F.) to produce products comprising transportation fuels, light gases, and slurry oil. The U.S. Pat. No. '697 is burdened by the complexity, cost, and technical viability of vacuum distilling a topped heavy crude to about 850.degree. F. and catalytic cracking the deasphalted oil to produce transportation fuels. This level of upgrading is too complex and required too large of a scale to be useful for oil field applications, and U.S. Pat. No. '697 selectively eliminates the majority of the material that could be used as fuel for a combustion turbine or internal combustion engine without further upgrading.
It is therefore an object of the present invention to provide a new and improved method of and means for upgrading hydrocarbons containing metals and asphaltenes wherein the disadvantages as outlined are reduced or substantially overcome.